1. Field of the Invention
The invention relates to a method for applying a liquid or pasty application medium on a running web of material, especially of paper or cardboard, and a paper machine with a device suitable for implementing the method.
2. Discussion of Background Information
U.S. Pat. No. 5,152,872 teaches that a one-sided application is made directly onto the fiber web with a roll application mechanism in a single-wire wet splitting device. In the application area, the wire loops around a roll supporting the web. The web lies on the side of the wire facing away from the roll. The roll application mechanism contacts the web in the looping area of the supporting roll. The roll application mechanism is a device having three rolls. Moreover, in the gap between the actual applicator roll and the web of material that has already been wetted, during the detachment process of the applicator roll surface from the web surface, roughening or picking occurs on the web surface. In this patent, it is not possible to apply an application medium in a twin wire area of a wet splitting device.
U.S. Pat. No. 4,793,899 discloses a similar applying device, as described above. In this patent, however, the application is made not in the wet splitting device but in the press section of a paper or cardboard machine. Moreover, the application mechanism applies the application medium with a machine-wide stream onto a combined application-and-press roll. Then this roll transfers the application medium to the web of material. This construction also causes picking on the web of material and in any event does not permit application in the twin wire area of a paper machine.
U.S. Pat. No. 5,622,599 teaches that a one-side application with spray nozzles from above directly onto the still-wet web in the wire section of a paper machine. The spray application occurs in the area of the straight wire course, in which wire washboard marks can form in part transverse with respect to the wire direction. In this manner, the spray application can occur irregularly on the surface of the web of material (viewed transversely with respect to the direction in which the wire moves). This construction also does not include application in a twin wire area. Moreover, a roll downstream from the spray application mechanism contacts the wetted web of material and results in the disadvantages picking problem described above.
An applying device is described in the as yet unpublished German patent applications DE 197 47 091 and DE 297 23 289, in which the application medium is applied to the still-wet web of material in an area supported by a roll. In so doing, the web of material can lie both between two wires and also on one wire only. If the surface of the web of material is covered by a wire, for example, the applying device feeds the application medium of the surface of the web of material through the wire. An additional application possibility is to apply the medium indirectly onto the web of material. Here, with the application mechanism, the medium is first applied to the surface of the roll supporting the wire. Through the rotation of the roll, the application medium is then fed through the supporting wire to the surface of the web of material. This application method is used, for example, if the surface of the web of material that faces away from the roll is not covered by a wire. The application media can be, for example, color coatings, starches, or also other substances. The disclosure content of the application specified above is incorporated in its entirety in this application.
Extremely high paper surface strengths are required particularly for heat-set and cold-set offset printing. In the prior art, the pigment particles of the color coating are only moderately anchored to the surface of the web of material. During printing, picking then occurs on the surface of the web of material, since the fiber and pigment particles are often held more strongly by the printing ink than by the web of material. In this process, fluffing or delaminating (i.e., separation of layers) partially occurs. This results in rather significant unevenness in the printed image.
The invention therefore provides for a method and a device in which an improved anchoring of the application medium on the web of material is achieved, so that its printability is improved and the technical outlay is reduced to the extent possible with respect to previous solutions.
This achieved in accordance with the invention disclosed herein.
It is now recognized that the application, for example, of a color coating with a suitable bonding agent (for example, mineral filling materials such a kaolin, clays, talcum, zeolite, CaCO3 or TiO2, etc.) in the twin wire area of the wet end section of a paper or cardboard machine exhibits a good anchoring with the web of material. This occurs because in the wet end section, the fibers lie relatively loosely at first in relation to their final condition after drying. Through the dewatering pressurexe2x80x94caused by dewatering strips or by the traction of the wire on a curved wire pathxe2x80x94water is more intensively driven from the fiber suspension. If, in this phase, the application medium is pressed with pressure through the wire into the fiber web via an applying device, the medium interlocks with the fibers of the surface of the web of material. Since some water is still present inside the web of material, the medium does not penetrate to the center of the web of material, but remains primarily in the outer layers. This results in a steep distribution of the medium in the z direction of the sheet of paper (i.e., over the flat cross-section), whereby the printability is significantly improved.
An applicator is used as an applying device in the twin wire area. This is a distributing pipe that is transverse with respect to the direction in which the wire moves and that has a slot-shaped nozzle that also extends in that direction. The medium is pressed out from the slot-shaped nozzle at a pressure of 0.05 to 0.5 barxe2x80x94preferably at 0.1 to 0.4 bar. Since this applicator is positioned near or in the immediate vicinity of the elements, such as dewatering strips or rolls, which support the surface of the wire, wire washboard marks, which extend transversely with respect to the direction in which the wire moves, are eliminated. A wire surface smoothed in this way makes possible a very close positioning of the applicator to the wire surface. Even direct contact with the surface of the wire is possible. This even spacing between the applicator and the surface of the wire that can be realized in this manner results in homogenous hydrodynamic conditions when viewed across the width of the machine. In this manner, a high degree of smoothness of application is achieved transversely with respect to the direction in which the wire moves. The application weight (dry weight of the application medium) ranges from 1 to 10 g/m2 per side, depending on the instance of utilization. The formed web of material has a solid content ranging from 5% to 50% or preferably ranging from 8% to 17%.
The application described above can be supported by a first addition of additives that have a surface bonding effect. These agents include, for example, starches, surface sizing agents, lattices, latex, and polyvinyl alcohols. These agents are applied jointly with the actual application medium or separately. Pre-dosing in the fiber material upstream from the headbox is also possible.
The present invention provides for a residual addition of the additives in an area of the paper machine in which the web of material already has a rather low moisture content high dry content. This leads to a higher retention of the additives in the web of material, since, owing to the low moisture content, less water flow from the web of material occurs. This applicator location is located preferably downstream from the first press element of the press section. The initial anchoring of the application medium in the twin wire area of the wet end section combined with a further application downstream with at least one additive results in an improved surface strength and thus an improved smoothness of the printed image. Furthermore, a reduced load of the water circuit of the paper machine is achieved since the additive remains more intensively in the web of material.
This combination of the measured application can also avoid the need for an additional sizing press or a coating machine.
The applicator mentioned above or spray nozzles can be used as applying devices for the additives.
The present invention provides for the application of fine fiber material in the twin wire area of the wet end section. In this manner as well, a surface bonding of the web of material is achieved. The fine fiber material is unintentionally washed out during dewateringxe2x80x94mainly from the surface of the web of material. Recirculating the fine fiber material causes it to be added back to the layer in the web of material from which it was lost. The fiber mat located between the wires thus functions as a filter. This fine fiber material can, for example, be recovered through fractionation of the wire water.
Additional embodiments of the invention may be ascertained by reviewing the present disclosure and the accompanying figures.
It is understood that the features of the invention specified above and to be explained below are not only applicable in the respectively indicated combinations, but also in other combination or individually, without exceeding the scope of the invention.